1. Field of the Invention
The present invention relates to a camera system, a camera and a lens apparatus attachable to a camera, particularly, having an automatic focusing (AF) function.
2. Description of Related Art
In a single-lens reflex camera to which a lens is interchangeable, a phase difference detection method is frequently used for detecting the focusing state of the image-taking optical system.
According to the phase difference detection method, a luminous flux incident through the lens is separated into two luminous fluxes, and the luminous fluxes are made incident on a pair of two AF sensors, respectively. The AF sensors photoelectrically convert the images formed by the separated luminous fluxes. The relationship between the outputs from the two AF sensors, that is, the distance between the two images differs according to the focusing state, that is, differs between front focus and rear focus, and focusing is performed by moving the focusing lens so that the distance between the images becomes the distance of the in-focus state.
That is, since the relationship between the shift amount between the two images and the image plane movement amount, the so-called defocus amount, depends on the image-taking optical system, the defocus amount is obtained from the shift amount. Then, the movement amount of the focusing lens is obtained from the defocus amount, and the focusing lens is moved, thereby obtaining an in-focus state.
In digital still cameras and digital video cameras in which images are captured in a two-dimensional image pickup device and the image signals thereof are outputted or recorded onto a recording medium, a contrast detection method and a focus detection method called a hill-climbing method are frequently used. In the contrast detection method, since the detectable range of the defocus amount is smaller than that of the phase difference detection method, focus detection is performed, for example, as represented below:
An example of the contrast detection method is a wobbling method. According to this method, image light is captured in the image pickup portion, and the high-frequency components of the outputted image signals are extracted. The highest value of the extracted signals is stored. Then, the focusing lens is moved in a certain direction, and image light is captured and the high-frequency components are extracted in a similar manner. When the highest value of the extracted signals becomes higher than the stored value, determining that the movement direction of the focusing lens is approaching the in-focus position, the current value is re-stored, and the focusing lens is moved in the same direction. When the current highest value is lower than the previous one, determining that the movement direction of the focusing lens is moving away from the in-focus position, the current value is re-stored, and the focusing lens is moved in the direction opposite to the previous direction. In this manner, extraction of the high-frequency components and comparison between the highest values are performed, and the focusing lens is moved so that the image plane is finally brought into the in-focus position.
Another example of focus detection methods using the contrast detection method is a trial-and-error method (including a whole area scanning method). According to this method, the focusing lens is sent to the close end or the infinity end and is moved assuming the sent position as the starting position, and image light is captured in the image pickup portion in a certain distance between the image planes. Then, the captured image light is photoelectrically converted into image signals, the high-frequency components of the image signals are extracted, and the highest value thereof is stored. This operation is repeated until the infinity end is reached when the starting point is the close end and until the close end is reached when the starting point is the infinity end. Then, the highest one of the stored plural highest values, that is, the focus position where the contrast is highest is obtained, and the focusing lens is moved to a position corresponding to the point.
Examples of cameras performing focus detection by the above-described two methods, the phase difference detection method and the contrast detection method, include cameras proposed in Japanese Patent Application Laid-Open No. H7 (1995)-43605 and Japanese Patent Application Laid-Open No. H9 (1997)-181954. Japanese Patent Application Laid-Open No. H9 (1997)-181954 proposes a camera performing hybrid AF by the phase difference detection method and the contrast detection method by use of the outputs of the image pickup device of an electronic still camera.
As another AF method, AF by an active method is known. According to this method, light is projected onto the object, the reflected light is detected by a sensor, and the defocus amount is calculated from the position of the reflected light.
However, when only the AF system using the phase difference detection method is used for digital still cameras, the ratio of the image pickup surface corresponding to one pixel of the AF sensor is high compared to that of film cameras. That is, the detection pixel is coarse and this decreases the accuracy of AF.
To solve this problem, it is necessary to decrease the magnification of the AF optical system and reduce the pixel pitch of the AF sensor. However, doing these is difficult in view of the structure and accuracy and increases the cost.
On the other hand, according to the AF system using only the contrast detection method, since the detectable range of the defocus amount is smaller than that of the phase difference detection method, when the object is largely out of focus, focus detection is difficult and the above-mentioned wobbling and whole area scanning are essential, which increases the time required for focus detection.
Moreover, according to the AF system using both the phase difference detection method and the contrast detection method, since lens driving by the phase difference detection method is performed and after it is confirmed that in-focus state is obtained, AF is performed in the vicinity thereof by the contrast detection method, highly accurate focusing control can be performed. However, compared to the case where only the phase difference detection method is used, additional time is required because of the time required for the lens driving by the contrast method. Moreover, if the lens driving direction is reversed while the lens is being driven by the contrast detection method, the camera operator may experience a sense of discomfort.
Further, according to the active method, there are cases where the defocus amount cannot be correctly calculated for some objects. In such cases, it is necessary to use another method in combination with the active method.